(a) Technical Field
The present invention relates to a system and a method for estimating generative braking of a vehicle, and more particularly to a system and a method for estimating regenerative braking of a vehicle to obtain accurate values of allowable regenerative braking and braking linearity by estimating allowable regenerative braking while taking into consideration the current status of a battery and a motor.
(b) Description of the Related Art
Regenerative braking technology applies, for the purpose of maximizing fuel ratio of an environment-friendly vehicle, a reverse torque to an electric motor using energy generated during braking for electric energy generation and stores the generated electric energy in a high voltage battery so as to enable the stored electric energy to initiate the vehicle, which can be applied to many environment-friendly vehicles. In a hybrid vehicle, in particular, regenerative braking plays a crucial role in fuel efficiency by accounting for about 30% of improvement in fuel efficiency as compared to general vehicles.
The motor and the battery employed in regenerative braking are restricted in charging and discharging under operating conditions. In the battery, the charging of the battery may be restricted under conditions such as very high or low state of charge, high or low temperature, or another problem of the battery. In the motor, the charging function of the motor may be restricted under conditions such as very high temperature of the motor, very fast rotating speed thereof, or another problem occurring in the motor.
For example, the charging of the battery may be restricted until an engine of a vehicle is warmed up in winter, then the charging function of the motor is also restricted and the regenerative braking is affected, so that the charging/discharging of the battery may be also restricted.
FIG. 1 shows characteristic curves of a motor when the charging function of the motor is restricted and not.
Referring to FIG. 1, the motor has a forward power and a forward torque as shown by the upper curve when the charging function of the motor is not restricted, but a forward power and a forward torque as shown by the lower curve when the charging function of the motor is restricted. In particular, the overlapped portion between the forward torque area of the upper curve and the lower curve indicates an area where the torque characteristics of the motor vary according to the conditions when the charging function of the motor is restricted or not, or the current status of the motor.
Since this change of the characteristics of the motor affects the regenerative braking, in the existing regenerative braking technology, based on when the charging function of the motor is not restricted, the braking feeling varies when the charging function of the motor is restricted. Thus, it would be desirable to provide a stable braking feeling even during the restricted charging function of the motor.
Moreover, if rotational speed of the motor varies in the forward power area, torque of the motor also varies (see FIG. 1). Thus, a wheel speed (i.e., an output speed of a transmission) does not vary when the transmission is released, but the rotational speed of the motor (i.e., an input speed of the transmission) varies sharply so that the torque of the motor also varies sharply. Then, the sharp change of the torque of the motor affects the rotational speed of the motor and the transmission is further released, and as a result, a driver feels sharp change of deceleration.
On the other hand, in a case of the existing estimation of a maximum regenerative braking, as illustrated in FIG. 2, a maximum regenerative braking value is estimated such that a minimum value (an output value from a comparator 3) of regenerative braking values which are outputted from a maximum regenerative braking map 1 and an active hydraulic brake (AHB) 2 is selected by comparing the two output regenerative braking values and the output regenerative braking values outputted from the AHB 2 and the comparator 3 are used.
In the case where the maximum regenerative braking value is estimated as described above, according to the existing art, the maximum regenerative braking map is set simply depending on a chargeable power of a motor without any consideration of the current status of the motor such as the restriction to the charging function of the motor.
Therefore, the maximum regenerative braking map outputs a meaningless estimate when the charging function of the motor is restricted, then the maximum regenerative braking (indicated as an excessive allowable regenerative braking in FIG. 3) is estimated, so that a value greater than a regenerative braking (indicated as an executive regenerative braking in FIG. 3) actually used in vehicle braking is transmitted to the AHB.
The AHB estimates a braking value, which is obtained by subtracting the regenerative braking (actual regenerative braking) estimated by the hydraulic control unit (HCU) from the demand braking by a driver, as a hydraulic braking value to perform the vehicle braking. In this case, the excessive regenerative braking is estimated, and as a result of the estimation, the executive hydraulic braking is estimated so as to be smaller than the actual hydraulic braking, so that total braking (which is the sum of the executive regenerative braking and the hydraulic braking) decreases, and a vehicle may slide as a result.
Further, in the related art, the maximum regenerative braking is estimated based on an input speed of the transmission. As illustrated in an area A indicated by yellow, a vehicle speed (output speed of the transmission) does not change when the rotational speed of a motor (the input speed of the transmission) varies due to the released transmission but a torque limit of the motor (maximum value of a motor torque) sharply varies. As a result, the actual regenerative braking value sharply varies, and the braking feeling becomes poor, thus leading to potential malfunction of the transmission (error in gear shifting) due to the sharp change of the motor torque, which may produce a negative feeling in a driver.